An important problem in the biology of mammalian cell growth concerns the regulation of messenger RNA metabolism. As a model for mammalian cell growth regulation, we have studied the serum induced proliferation of contact inhibited fibroblast, Balb C/3T3 cells. Since mRNA molecules are associated with proteins (mRNP) through most of their life history in the cell, our understanding of mRNA metabolism must ultimately be expressed at the ribonucleoprotein level of organization. The hypothesis underlying our experimental approach has been that the mRNA-protein interactions are important determinants in the post-transcriptional regulation of mRNA metabolism. Our studies have indicated an increased stability of mRNA during the transition from resting to serum induced growth. During this transition there is a characteristic modulation in the size and cytoplasmic distribution of poly(A) segment. Further, the rate of protein synthesis is increased five fold from resting to growing cells. The inhibition in resting 3T3 cells appears to be largely at the translational initiation step. Proposed studies are aimed at furthering our understanding of the possible role of mRNP proteins in translational regulation. In particular, the proportions of poly(A) containing and poly(A) deficient mRNA molecules from resting and growing cells will be determined on the basis of buoyant densities of RNP particles. The binding sites of major mRNP proteins on mRNA will be located by partial S1 nuclease digestion and selection of 3' poly(A) ends of mRNP by oligo d(T) cellulose chromatography. Translational efficiencies of the polysomal and free mRNP will be compared in cell free protein synthesizing systems. Metabolic properties of mRNP proteins and initiation factors, purified from ribosome eluate and heparin-cellulose chromatography will be compared by two-dimensional resolution. These studies will therefore help explain the nature of post-transcriptional regulation of messenger RNA with respect to the proliferative state of 3T3 cells and may be useful in understanding mammalian cell growth regulation.